The problem was we have just implemented Endian as a UTM solution (about a month ago). The restart of the Zimbra server kicked in a https_proxy configuration that had not come into effect before, and did today after the server was rebooted.

The bottomline was that Zimbra was trying to make connections over port 7071 which is not regularly allowed to go through squid. The solution was to eliminate the https_proxy and Outlook was able to send emails once again. This is the best solution because it doesn´t make sense to send this traffic through squid when it is all local traffic.